Background: Post-traumatic stress disorder (PTSD) is highly prevalent in patients with a psychotic disorder. Because a PTSD diagnosis is often missed in patients with psychosis in routine care, a valid screening instrument could be helpful. Aims: To determine the validity of the Trauma Screening Questionnaire (TSQ) as a screening tool for PTSD among individuals with psychotic disorders. Method: Among 2608 patients with a psychotic disorder, the rate of trauma exposure was determined and the TSQ was administered to screen for PTSD. PTSD status was verified in 455 patients using the Clinician-Administered PTSD Scale (trial registration: ISRCTN 79584912). Results: Trauma exposure was reported by 78.2% of the 2608 patients. PTSD prevalence was estimated at 16% (95% CI 14.6-17.4%) compared with 0.5% reported in the patients' clinical charts. A TSQ cut-off score of six predicted PTSD with 78.8% sensitivity, 75.6% specificity, 44.5% correct positives and 93.6% correct negatives. Conclusions: The TSQ seems to be a valid screening tool for PTSD in patients with a psychotic disorder.